Deacidizing gaseous effluents such as, for example, natural gas and combustion fumes, is generally carried out by washing with an absorbent solution. The absorbent solution allows the acid compounds present in the gaseous effluent (H2S, mercaptans, CO2, COS, SO2, CS2) to be absorbed.
Deacidizing these effluents, notably decarbonation and desulfurization, imposes specific requirements on the absorbent solution, in particular thermal and chemical stability, notably towards the impurities in the effluent, i.e. essentially oxygen, SOx and NOx. The oxygen can also come into contact with the absorbent solution without being necessarily present in the gaseous effluent to be treated, for example in case of an accidental air inflow in the absorbent solution storage tanks.
Currently, the most commonly used absorbent solutions are aqueous alkanolamine solutions. Documents FR-2,820,430 or U.S. Pat. No. 7,056,482 provide gaseous effluent deacidizing methods.
It is however well known to the person skilled in the art that the amines used for deacidizing a gaseous effluent involve the major drawback of being corrosive to the steels the installations are made of. Indeed, the facilities for implementing deacidizing methods are commonly made from low-alloy steel. Low-alloy steel corresponds to a steel with a proportion of each additional element below 5 mass % and whose manganese content is below 1 mass %. Corrosion designates the alteration of the steel by chemical reaction with an oxidant (predominantly dioxygen and the H+ cation).
In order to overcome the problem of equipment corrosion, since it is not possible to decrease the amine concentration and/or to decrease the CO2 filling ratio and/or to decrease the temperature, one solution consists in adding to the absorbent solution compounds referred to as corrosion inhibitors or anti-corrosion additives.
The most efficient inhibitors belong to the inorganic compounds family. Additives based on antimony or vanadium are mentioned in patents GB-1,393,302, GB-1,360,836, U.S. Pat. No. 3,959,170 or EP-0,043,525. Copper-based additives are mentioned in U.S. Pat. No. 4,071,470, U.S. Pat. No. 4,440,731, U.S. Pat. No. 4,477,419, U.S. Pat. No. 4,595,723 and U.S. Pat. No. 4,596,849. Other heavy metal salts also have good corrosion inhibiting properties, as described in U.S. Pat. No. 4,452,764, U.S. Pat. No. 4,100,099, U.S. Pat. No. 4,100,100 and U.S. Pat. No. 4,431,563.
Another family of organic inhibitors is also often provided. Patent US-2011/0,300,044 describes the use of imidazole, dodecylamine or morpholine. The efficiency of these types of corrosion inhibitors is however lower in relation to the inorganic inhibitors.
These corrosion inhibitors however involve some major drawbacks. Inorganic inhibitors most generally contain heavy metal salts with often a high level of toxicity to the environment. On the other hand, they tend to promote amine degradation. Organic inhibitors generally have a more neutral effect as regards amine degradation, but their efficiency against corrosion is also lower.
A second major drawback is that the amines of the absorbent solution degrade under the conditions of use, in particular by chemical oxidation reaction with some constituents of the gas to be treated.
Specific solutions also exist to overcome this degradation problem. Since it is not possible to limit or to suppress the presence of oxygen in the absorbent solution, one solution consists in adding to the absorbent solution compounds whose purpose is to prevent or to limit amine compound degradation phenomena, notably the degradation generated by oxidation phenomena. These compounds are commonly referred to as degradation inhibiting agents or additives. The main modes of action known for degradation inhibiting agents are, depending on the nature thereof, a reaction of reduction and/or capture, trapping and/or stabilization of the radicals formed in the absorbent solution so as to limit or to prevent or to interrupt the degradation reactions, notably chain reactions.
U.S. Pat. No. 5,686,016 mentions additives used for limiting the degradation of absorbent solutions used for deacidizing natural gas, in particular oximes.
U.S. Pat. No. 7,056,482 mentions additives used for limiting the degradation of absorbent solutions used for CO2 capture, such as alcohols, thiosulfates, phenolic amines, aromatic amines and sulfites.
Patent WO-11,064,469 mentions additives derived from thiazole or oxazole for limiting the degradation of an amine solution used for deacidizing a gaseous effluent. For the same application, patent WO-11,064,470 mentions the use of pyrimidine or triazine derivatives; patent WO-09,156,618 describes inhibitors having a thiocarbonyl function.
These degradation inhibitors generally have a neutral or negative effect as regards corrosion. Furthermore, their efficiency is substantially reduced in the presence of dissolved metal cations resulting from the corrosion of the equipments.
From the viewpoint of the user of the gaseous effluent deacidizing method, it is therefore always necessary to combine an anti-degradation solution and an anti-corrosion solution.
U.S. Pat. No. 4,477,419 mentions a method allowing corrosion and degradation to be decreased by the presence of copper ions in solution. For smooth operation of the method, it is however necessary to add to the deacidizing method stages of continuous removal of the dissolved iron and of the amine degradation products through mechanical filtration and chemical treatments on activated coals and ion-exchange resins. It is also necessary to take specific steps so as to prevent metallic copper deposition on the steel equipments, which might lead to corrosion increase through galvanic coupling effect. These stages substantially complicate the deacidizing operation and they increase the investment and operating costs.
Patent FR-2,938,453 aims to reduce the degradation of an absorbent solution used in a gas deacidizing facility by avoiding contact between the solution and low-alloy steel surfaces. This solution affords the advantage of using no chemical additive in the deacidizing solvent. Its performances are however limited. In particular, as shown in Example 1 hereafter, the amine degradation is in fact not sufficiently reduced when the amine content is above 30 wt. %.
The object of the present invention thus is to provide a solution allowing to limit the degradation of an amine solution, notably for a concentration greater than or equal to 30 wt. %, used for deacidizing a gaseous effluent while preventing facility corrosion problems.